Dimeric ethyltin(IV)–dibromide–hydroxide–N,N-dimethylformamide

The title compound exhibits the typical rhomboid-like four-membered Sn–OH ring found in all dimeric molecules of monoorganotin(IV)–dihalide–hydroxides, with acute bond angles at the Sn atom, obtuse bond angles at O atoms and shorter Sn—O bond lengths trans to the ethyl groups.

The title compound, belonging to subclass ii, was found accidentally as a hydrolysis product of humid air during an attempt to synthesize a complex of ethyltin(IV)-tribromide with DMF and represents the first structurally characterized monoorganotin(IV)-hydroxide-dihalide with bromine as the halide.

Structural commentary
The title compound crystallizes in the monoclinic space group P2 1 /c, as was unambiguously confirmed from systematic absence conditions.The unit cell contains two dimeric centrosymmetric molecules (Fig. 1), resulting in half a molecule in the asymmetric unit.The molecule exhibits the typical structural features of the monoorganotin(IV)-dihalide-hydroxides, i.e. two octahedrally coordinated Sn atoms are linked together via two bridging hydroxide groups whereby a planar four-membered Sn-OH ring results.
This Sn-OH ring (Fig. 2) has a characteristic rhomboid-like shape with acute [70.01 (8) � ] angles at the Sn atoms, obtuse angles [109.99 ( 8) � ] at the O atoms and two distinct different tin-oxygen bond lengths [2.071 (2) and 2.1461 (1) A ˚], the shorter of which is opposite to the organic group.This kind of bond-length shortening, designated in the literature as transstrengthening (Paseshnitchenko et al., 1985;Buslaev et al., 1989), is typically found in the case of monoorganotin(IV) compounds with tin in a sixfold octahedral coordination.
Four-membered Sn-OH rings are structure-dominating features in many organic and inorganic tin(IV) compounds.Thus, they occur, for example, in the dimeric diorganotin(IV)halide-hydroxides, [R 2 SnHal(OH)] 2 , with trigonal-bipyramidally coordinated Sn atoms.There the bond angles are in the same order; different Sn-O bond lengths, however, result from the axial and equatorial positions of the hydroxide groups within the trigonal-bipyramidal coordination of the Sn atoms (cf.Reuter, 2022).A somewhat different geometry is observed in the case of the four-membered Sn-OH rings of the dimeric tin(IV)-trihalide-hydroxide-aqua complexes, [SnHal 3 -(OH)(H 2 O)] 2 , where the Sn atoms are also octahedrally coordinated.These compounds constitute the pure inorganic equivalents of the class of compounds discussed here with an additional halide atom instead of the organic group R. In analogy to the dimeric monoorganotin(IV)-dihalide-aquacomplexes, these inorganic counterparts can be divided into similar subclasses.For Hal = Br, the structures of only two polymorphs (Howie et al., 2005;de Lima et al., 2010) of a hydrate (subclass iii), with 3.5 additional water molecules, are actually known.In both, the dimeric molecules are noncentrosymmetric and the Sn-OH rings are not planar, but only slightly buckled.Nevertheless, these rings exhibit a geometry  The C-C distance [C1-C2 = 1.485 (5) A ˚] in the ethyl group is to some extent shorter than the value of 1.513 (14) A evaluated by Allen et al. (1987) for the mean distance between two sp 3 -hybridized C atoms.This deviation is probably caused by atom vibration, as indicated by the displacement ellipsoids (Fig. 1).The Sn-C distance [Sn-C = 2.228 (2) A ˚] is enlarged compared to the sum (2.15A ˚) of the normal covalent radii (Cordero et al., 2008) of tin (1.39A ˚) and carbon (0.76 A ˚), but is in the same order of magnitude as the Sn-C bond length (Lecomte et al., 1976).Much shorter tin-carbon bonds [2.139 (4) and 2.130 (4) A ˚] have been reported for the corresponding DMF compound with R = i Bu and Hal = Cl (Reuter & Ye, 2013).
Both tin-bromine bonds are of different lengths with the longer one [2.6360(3) A ˚] in the case of the in-plane (ip) Br1 atom and the shorter one [2.5893(4) A ˚] in the case of the outof-plane (oop) Br2 atom.The reason for this obviously arises from the fact that the first is involved in a hydrogen bond with   the hydroxide group of a neighbouring molecule (see below), while the second is only involved in van der Waals interactions.It is notable that both values are markedly longer (0.069 and 0.080 A ˚) than the tin-bromine distances in the above-mentioned tin(IV)-tribromide-hydroxide-aqua-hydrates [mean Sn-Br ip = 2.509 (5) A ˚, 8 data points; mean Sn-Br oop = 2.567 (14) A ˚, 4 data points].
The coordinated DMF molecule is almost planar, as the distances of the O, C and N atoms from the least-squares plane indicate (Fig. 3).The coordinative bond has a length of 2.177 (2) A ˚, while the bond angle at the O atom is 126.2 (2) � .Both values differ significantly from the corresponding values [2.210 (3)/2.202(4) A ˚and 120.8 (3)/124.8(4) � ] observed in the noncentrosymmetric molecules of [ i BuSnCl 2 (OH)(DMF)] 2 (Reuter & Ye, 2013).The angle between the least-squares plane through the non-H atom of the DMF molecule and the Sn-O DMF bond length is 3.12 (8) � .
Structural distortion of the DMF molecule as a result of its coordinative bond to the Sn atom is well expressed and concerns not only the bond lengths but also the bond angles.Structural data for pure DMF have been determined twice (Borrmann et al., 2000;Ratajczyk et al., 2019) under normal pressure and at a temperature of 100 K.Both crystallize in the triclinic space group P1, with two different molecules in the asymmetric unit.As the individual structure parameters within both molecules and between the different measurements differ to some extent, in the following, the mean values of each four data points are used.Most notable are the changes in bond lengths: thus, the carbon-oxygen distance increases by 0.031 A ˚from 1.229 (2) A ˚in pure DMF to 1.260 (4) A ˚in the coordinated molecule; simultaneously, the carbon-nitrogen distance decreases by 0.038 A ˚from 1.339 (2) to 1.301 (4) A ˚, while the distances between the methyl C atoms and the N atoms remain mostly unaffected [cis-CH 3 -N(pure/coordinated) = 1.453 (2)/1.457(6) A ˚and trans-CH 3 -N(pure/coordinated) = 1.454 (2)/1.461(5) A ˚].The greatest changes of the bond angles are observed for O-C-N, decreasing by 2.3 � from 125.4 (2) � in pure DMF to 123.1 (3) � in the coordinated molecule, and to a smaller extent (0.8 � ) for CH 3 -N-CH 3 , increasing from 117.2 (3) to 118.0 (3) � .The changes of the CH-N-CH 3 angles range from À 0.4 to À 0.5 � .

Supramolecular features
In the solid, hydrogen bonds exist between the hydroxide groups and the Br1 atoms of adjacent molecules, as the spacefilling model (Fig. 4) using the van der Waals radii of Mantina et al. (2009) indicates.The resulting chain-like arrangement of the hydrogen-bonded molecules (Fig. 5) takes place in the direction of the crystallographic a axis.With a donor-acceptor distance of 3.283 (2) A ˚between the Br and O atoms, they rank as strong.The bridging angle at the H atom is 164.8 � .As the second Br atom (Br2) does not take part in any hydrogen bonds, the interactions between the individual chains are confined to van der Waals contacts (Fig. 6).

Synthesis and crystallization
In a fumehood, 0.39 g (1 mmol) of ethyltin(IV) tribromide, C 2 H 5 Br 3 Sn, prepared from ethyltin(IV) trichloride via halide exchange with an excess of potassium bromide in dry acetone was mixed with 2 ml N,N-dimethylformamide (DMF) on a petri dish with a glass lid.Crystal formation was checked every day using an optical microscope.The first crystals of the title compound appeared after two weeks.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.The positions of all H atoms were clearly identified in difference Fourier syntheses.Those of the organic groups were refined with calculated positions (-CH 3 = 0.96 A ˚, -CH 2 -= 0.97 A ˚and -CH-= 0.93 A ˚) and common U iso (H) parameters for each individual group.The position of the H atom of the OH group was refined with a fixed O-H distance of 0.96 A ˚before it was fixed and allowed to ride on the parent O atom with an isotropic displacement parameter.(Bruker, 2009), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), DIAMOND (Brandenburg, 2006), Mercury (Macrae et al., 2020) and publCIF (Westrip, 2010).

Figure 1
Figure 1 Displacement ellipsoid plot of the dimeric centrosymmetric molecule found in the crystal of [EtSnBr 2 (OH)•DMF] 2 , showing the atom numbering of the asymmetric unit.With the exception of the H atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn with displacement ellipsoids at the 40% probability level.

Figure 2
Figure 2 Displacement ellipsoid plot of the centrosymmetric four-membered tinoxygen ring of the [EtSnBr 2 (OH)•DMF] 2 molecule, highlighting selected bond lengths (A ˚), angles ( � ) and distances (A ˚) from the Sn-O reference plane in square brackets.With the exception of the H atoms, which are shown as spheres of arbitrary radius, all other atoms are drawn with displacement ellipsoids at the 40% probability level.For clarity, ethyl groups are stripped down to the Sn-C bonds drawn as shortened sticks.Intermolecular O-H� � �Br hydrogen bonds are indicated as dashed sticks in brown.Descriptors trans and cis refer to the position of the corresponding bonds with respect to the tin-carbon bond of the ethyl group.

Figure 3
Figure 3Displacement ellipsoid plot of the DMF molecule, with selected bond lengths (A ˚), angles ( � ) and distances (A ˚) from the least-squares plane through the non-H atoms in square brackets.The dative Sn� � �O bond is indicated as a shortened stick.

Figure 4
Figure 4 Space-filling model of the [EtSn(OH)Br 2 •DMF] 2 molecule, showing the overlap of the H and Br atoms in the region of the hydrogen-bridging bond.These atoms are visualized as truncated two-coloured spheres.Atom colours and van der Waals radii (A ˚) are as follows: Br = brown/ 1.83, H = white/1.10,C = grey/1.70,O = red/1.52,N = blue/1.55and Sn = brass/2.17.

Figure 5 6
Figure 5 Stick-model showing in detail the chain-like arrangement of the [EtSn(OH)Br 2 •DMF] 2 molecules resulting from intermolecular O-H� � �Br hydrogen bonds (red dashed sticks).The image shows three complete molecules with their hydrogen bonds to neighbouring molecules.Two-coloured sticks based on atom colours are as follows: Br = brown, H = white, C = grey, O = red, N = blue and Sn = brass

Table 1
Experimental details.
Computer programs: APEX2 and SAINT meinschaft (DFG) and Open Access Publishing Fund of Osnabru ¨ck University.